Central Dogma and the Genetic Code Chapter 14 Central Dogma and the Genetic Code

Proteins are the links between genotype and phenotype. Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation. The information content of genes is in the form of specific sequences of nucleotides in DNA. The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins.

DNA RNA Review sections 4.3 and 4.4 in your textbook Transcription is the synthesis of RNA using information in DNA. Transcription produces messenger RNA (mRNA). Translation is the synthesis of a polypeptide, using information in the mRNA. Ribosomes are the sites of translation.

Central Dogma Transcription Translation DNA RNA Protein Figure 14.UN01

RNA is the bridge between DNA and protein synthesis. RNA is chemically similar to DNA, but RNA has a ribose sugar and the base uracil (U) rather than thymine (T). RNA is usually single-stranded. 5 5

In prokaryotes, translation of mRNA can begin before transcription has finished. DNA TRANSCRIPTION mRNA TRANSLATION Polypeptide Ribosome (a) Bacterial cell 6

primary transcript Nuclear envelope Pre-mRNA mRNA DNA RNA PROCESSING TRANSCRIPTION TRANSLATION Polypeptide Ribosome (b) Eukaryotic cell Nuclear envelope DNA primary transcript TRANSCRIPTION Pre-mRNA mRNA RNA PROCESSING (b) Eukaryotic cell Pre-mRNA In eukaryotes, the nuclear envelope separates transcription from translation. Eukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNA. A primary transcript is the initial RNA transcript from any gene prior to processing. Eukaryotic mRNA must be transported out of the nucleus to be translated. (b) Eukaryotic cell 7

DNA template strand 3 5 A C C A A A C C G A G T T G G T T T G G C T Figure 14.5 DNA template strand 3 5 A C C A A A C C G A G T T G G T T T G G C T C A 5 3 3 5 TRANSCRIPTION U G G U U U G G C U C A mRNA 3 5 Codon Codon Protein Trp TRANSLATION Amino acid Phe Gly Ser The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words. The words of a gene are transcribed into complementary nonoverlapping three-nucleotide words of mRNA. These words are then translated into a chain of amino acids, forming a polypeptide. During transcription, one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript. The template strand is always the same strand for any given gene. During translation, the mRNA base triplets, called codons, are read in the 5 to 3 direction. Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide. Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced. Codons are read one at a time in a nonoverlapping fashion. 8

What amino acid does AGG code for? Arg (arginine) What amino acid does CAU code for? His (histidine) What amino acid does GAC code for? Asp (asparagine) Second mRNA base U C A G UUU UCU UAU UGU U Phe Tyr Cys UUC UCC UAC UGC C U Ser UUA UCA UAA Stop UGA Stop A Leu UUG UCG UAG Stop UGG Trp G CUU CCU CAU CGU U His CUC CCC CAC CGC C C Leu Pro Arg CUA CCA CAA CGA A Gln CUG CCG CAG CGG G First mRNA base (5 end of codon) Third mRNA base (3 end of codon) AUU ACU AAU AGU U Asn Ser AUC IIe ACC AAC AGC C A Thr Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation. AUA ACA AAA AGA A Lys Arg AUG Met or start ACG AAG AGG G GUU GCU GAU GGU U Asp GUC GCC GAC GGC C G Val Ala Gly GUA GCA GAA GGA A Glu GUG GCG GAG GGG G 9

How many codons are there fore arginine (arg)? 6 (Redundant) Do any codons specify more than one amino acid? No. The code is not ambiguous. Second mRNA base U C A G UUU UCU UAU UGU U Phe Tyr Cys UUC UCC UAC UGC C U Ser UUA UCA UAA Stop UGA Stop A Leu UUG UCG UAG Stop UGG Trp G CUU CCU CAU CGU U His CUC CCC CAC CGC C C Leu Pro Arg CUA CCA CAA CGA A Gln CUG CCG CAG CGG G First mRNA base (5 end of codon) Third mRNA base (3 end of codon) AUU ACU AAU AGU U Asn Ser AUC IIe ACC AAC AGC C A Thr Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation. The genetic code is redundant: more than one codon may specify a particular amino acid. But it is not ambiguous: no codon specifies more than one amino acid. AUA ACA AAA AGA A Lys Arg AUG Met or start ACG AAG AGG G GUU GCU GAU GGU U Asp GUC GCC GAC GGC C G Val Ala Gly GUA GCA GAA GGA A Glu GUG GCG GAG GGG G 10

The genetic code is nearly universal! The genetic code is nearly universal, shared by the simplest bacteria and the most complex animals. Genes can be transcribed and translated after being transplanted from one species to another.